Sauté Onions. Set the Instant Pot to the sauté function. Cook the onions in butter and olive oil until deeply caramelized.
Pressure Cook. Add thyme, bay leaves, beef broth, salt, and pepper to the pot. Pressure cook for 5minutes to intensify the flavors. Allow the pressure to release naturally for 5 minutes, and then quickly release remaing pressure.
Prepare Baguette. While the soup cooks, slice the baguette and toast the slices until golden brown.
Serve. Ladle the soup into oven-safe bowls, top with a toasted baguette slice, and sprinkle with grated Gruyere cheese. Broil until the cheese is bubbly and golden.
Garnish and Enjoy. Garnish with fresh thyme, if desired, and serve the Instant Pot French Onion Soup hot.